RO Unit Filter change

I have an Aquatic Life 100gpd RO unit. I have made approximately 500 to 750 gallons of RO water with it during the first 4 months I have owned it.

It was producing a TDS of about 4 in the beginning. It is now 20+. I replaced the pre-filter, carbon block and mixed resin cartridge. It is now reading 18 TDS. The pre-filter was dirty but not bad. Neither was the mixed resin.

My water at the tap is about 170 TDS. I am using a handheld TDS-3 meter that is also 4 months old.

What do you think the possible cause is? Maybe I don't have the cartridges in correctly?

 

What PSI is the tap water going into it?

 

I don't know.

 

Well that can make a big difference right there. These systems rely on pressure to work properly. Any way you can get a gauge on there? Or another indicator would be how long it takes to make a gallon of water. Any rough ideas?

 

It takes about 20 minutes or less to make a gallon

 

Several things affect how well a RO/DI system functions. Water pressure is the most important and all RO or RO/DI systems should have an inline pressure gauge to monitor incoming pressure after the sediment and carbon block filters. Membranes for the most part will not function below 40 psi and they love pressures in the 60+ range up to around 100 psi, the higher the pressure the better they work, both GPD and rejection rate or removal efficiency.
The second thing is water temperature. Membranes are rated at 77 degrees F as a baseline for comparing one to another. That does not mean they work best at 77 degrees as they don't, it just allows you to compare a Dow Filmtec to a GE or Applied or whatever brand and GPD. Colder water actually treats better and will give you lower TDS from the membrane making DI last longer but since cold water is more dense it slows the GPD or production rate. Warm water on the other hand increases the flow rate, GPD, but the TDS will go up and DI life will be shortened so it is best to use cold water only.

The waste ratio affects how much you can make and how long the RO membrane will last. Some try to cut the waste below the recommended 4:1 which increases pressure to the membrane and makes more GPD but it also shortens RO membrane life since it is not being sufficiently flushed.

Softened water greatly increases RO life since the softener does much of the work for the membrane, kind of like pretreatment.

At 50 psi, 77 degree water temp, a 4:1 waste ratio and 250 TDS softened tap water you can expect to make right at 3 gallons per hour best case with a 75 GPD RO membrane which is the most popular size. 100 GPD Dow Filmtec membranes are the worst possible choice for a reef system since they are only 90% rejection versus 96-98% rejection for the 75 GPD version. If the pressure is higher the GPD will be higher or if the water colder it will be slower or any combination of the above can vary the production rate and the RO only TDS.

DI life and final TDS are entirely dependent on how well your RO membrane is working, RO only TDS, how much water you make and if you have CO2 in your water.

You need to know these things to troubleshoot your RO/DI system:

Tap water TDS

Tap water pressure

Tap water temperature

Exact measured waste ratio

Soft water or not?

CO2 in your water, this can be calculated if you know the alkalinity and the pH of your tap water

RO only TDS after the RO membrane but before the DI

Final RO/DI TDS

 

I appreciate the thorough reply. Unfortunately I can't control temperature or pressure. I elected not to buy the version that has a pump on it. If Temp and pressure are the only variables I will have to put up with 20TDS.

 

One simple thing to check...sorry if this is too obvious...but wanted to throw it out there...is make sure the container you are using is clean...ie...are you making saltwater in the same bucket that you are putting your ro water in. That might be a problem there..

Also is the unit RO/DI or just RO? If not using the DI, look at getting a DI add on unit..that might be the ticket to getting to 0 TDS. I have a RO/DI unit that I run with about 40 PSI..sometime mid 30's depending on water pressure that day...and I am getting 0 TDS consistently for 8-12 months at a time - I make a lot of water too..I have a 250 gallon display. I have the bulk reef supply RO/DI unit with the pressure guage and TDS meter on it.

 

Don't settle for a TDS of 20 when your tap water TDS is only 170. Thats a rejection rate of only 88% which is horrendous.

Is that after the DI or RO only? If its after the DI, what is your RO only TDS before the DI? If the membrane is working correctly it should be somehwre between 3 and 7 at most and your DI should last a long time.

I know you can't do anything about the temperature but you can monitor pressure drop or headloss due to plugged sediment and carbon block filters with a pressure gauge, all systems should have one, it a basic troubleshooting tool just like a TDS meter. The greater the pressure loss the less efficient the RO membrane will be.

Remember, sedimen tand carbon block filters have very little to absolutely nothing to do with TDS, they are there to protect the RO membrane from particulates and chlorine. The membrane does 98% of the TDS removal and the DI does the rest so you want the membrane to be as clean and efficient as possible. If the membrane is not working well a $40 or so investment will get you a very good, treated and tested 90 GPD membrane which will do better than 98% rejection guaranteed and would pay for itself quickly in DI savings.

 

Do you have a brand or source you would recommend? The Aquaticlife replacement is over $100.